"I don't think so," Rosenstein said when asked if Herbalife was a pyramid scheme during a panel discussion on activist investing at the Milken Institute's Global Conference in Los Angeles Tuesday.

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Rosenstein noted that Herbalife was one of his first investments after setting up hedge fund firm JANA in 2001. He said he pushed them to go private while he was long the stock. Herbalife is a public company today.

Others who have bet on the value of Herbalife's stock to appreciate include Dan Loeb of Third Point, Carl Icahn of Icahn Enterprises, and George Soros of Soros Fund Management. Ackman's Pershing Square Capital Management is famously short the stock.